1. Field of the Invention
The present invention relates to electro-optic devices, and in particular, to polarization independent broadband optical modulators and switches for wideband fiberoptic networks.
2. Background of the Related Art
A balanced bridge optical switch has two input and two output waveguides. Two ½lc (half a coupling length) 3-dB (i.e., 50:50 power splitter) directional couplers and a phase modulated interferometer waveguide pair are used. Note that “lc” is the characteristic coupling length of a directional coupler, which is the length of the directional coupler necessary to transfer substantially all the power from a first waveguide to a second waveguide. The input 3-dB coupler is used to equally divide an input signal received by an upper input waveguide into both the upper and lower waveguides prior to entering an interferometer section of the balanced bridge optical switch. An output ½lc directional coupler rejoins the electro-optical phase modulated signal from the interferometer section back into the upper output waveguide for a “straight-thru” path (i.e., “on” or (=) bar state) or the lower output waveguide in the “cross-over” path (i.e., “off” or switched (×) state). If the optical path lengths of the two waveguides are identical in the middle interferometer section, the two optical waves arrive at the output (½lc) 3-dB coupler section and recombined coherently producing an optical signal that transfers all into the lower waveguide or crossover path. However, if there is a 180° phase difference between the two optical path lengths in the interferometer section, the two optical waves will recombine and transfer back to upper waveguides or the straight through path. Thus, electro-optical phase modulation of the two optical waves in the interferometer section results in an amplitude modulation at each of the output waveguides and the device can be used as a 1×2 or a 2×2 optical switch.
FIG. 1 illustrates a related art polarization-independent 2×2 electro-optic switch 100 based on a balanced-bridge interferometric waveguide structure. As shown in FIG. 1, the related art switch 100 includes upper and lower waveguide patterns 120, 130 formed in a X-cut Z-propagation lithium niobate (LiNbO3) electro-optic substrate 110. The switch 100 includes a 3 dB-directional coupler section 142 (50:50 power splitter) having an interaction length of LB=½Ldc, an interferometer section of length Le, and an output 3 dB-directional coupler section 146 having an interaction length of LB=½Ldc, where LB=½Ldc, Ldc equals combined length of the two couplers, and lc equals characteristic coupling length of the directional coupler. Note that “interaction length” and “effective coupling length” are used interchangeably to describe the actual length of the waveguides in a directional coupler over which a signal may couple from a first waveguide to a second waveguide. Also note that the effective coupling length may be described in terms or units of characteristic coupling length lc. Thus a range of effective coupling lengths or interaction lengths for a directional coupler can be indicated as 0.75lc to 1.1lc. The input and output 3 dB-directional coupler sections operate as 50:50 power splitters and are preferably identical with a combined length where lc is the characteristic coupling length of the switch 100. As shown in FIG. 1, the upper waveguide 120 receives an input optical signal Iin. Under straight-thru switch operations, the input signal Iin received by the upper waveguide 120 exits from the waveguide 120 as an output signal Iupper (=), and in cross-over switch operations, the input signal Iin enters the upper waveguide 120 but exits through the lower waveguide 130 as Ilower (×).
For the related art switch 100, the upper and lower waveguides 120, 130 are single mode for each of two polarizations (TE, TM), and therefore support one TE mode and one TM mode each, where the “TE mode” is the transverse electric field mode, and the “TM mode” is the transverse magnetic field mode. A normalized applied voltage V applied in the interferometer section 144 is applied with an electric field in the Y-direction (Ey) that induces a differential propagation constant Δβ (“Δβ”) between the two interferometric sections of the upper and lower waveguides 120, 130 of length Le via the linear electro-optic effect. The length Le is the length of the electrodes 150, 152,154. Electrodes 150, 152, 154 are arranged in a push-pull configuration to maximize the electro-optically induced Δβ between the upper and lower waveguides 120, 130 in the interferometer section 144. Thus, the electrode 152 receives the normalized applied voltage V and the electrodes 150 and 154 receive a ground potential. As shown in FIG. 1, the placement of the electrodes 150, 152, 154 maximizes the E-field along the Y-axis inside the waveguides.
For the X-cut substrate 110, when the waveguide propagation direction in the waveguides 120, 130 is along the Z-axis (optic axis), both the TE and TM modes see the same ordinary index (no). Thus, both the TE and TM polarization modes are nearly degenerated and will behave in approximately the same way. The electro-optic (“EO”) interaction for the TE and TM modes with the Ey field in the interferometer section are via EO coefficients that are equal but opposite in sign (i.e., r22, −r22) in the lithium niobate substrate 110. Therefore, the magnitudes of the Δβi for both the TM and TE modes are the same in the interferometer section 144 where Δβi is the difference in the propagation constants between the two waveguide pair in the middle “interference” section, and Δβdc is the difference in the propagation constants between the waveguide pair in the “input” and “output” directional coupler sections. In other words, the EO interactions in the interferometer section 144 in the TE mode is proportional to +r22 (Ey) and the TM mode is proportional to −r22 (Ey) as the corresponding change in the propagation constants is via the electric field in the Y-axis direction.
In FIG. 2A, Icross-over and Istraight-thru conditions are illustrated for the related art switch 100, where optical power is a vertical axis and the ratio of normalized applied voltage to Vπ, which is the voltage required to cause a 180° phase shift between the two arms of the interferometer V/Vπ, is a horizontal axis. As shown in FIG. 2A, the input signal Iin is output as Icross when V/Vπ is equal to −4, −2, 0, 2, 4 and Iin is output as Iupper (I straight-thru) when V/Vπ is equal to −3, −1, 1, 3.
In FIG. 2A, the optical power is illustrated from 0 to 1 corresponding to an on (=) or off (×) state of the switch 100. For a high performance, low-crosstalk switching device, the input and output directional couplers 142, 146 of the switch 100 must behave as 3 dB-couplers for both the TE and TM modes simultaneously. In this case of zero (0) voltage, both the TE and TM modes entering an input port of the upper waveguide 120 will exit a lower output “cross-over” port (×) of the lower waveguide 130. When voltage is applied to the interferometer section 144 of the switch 100 with an electric field in the Y-axis direction, the EO induced change in the waveguide indices for the TE and TM modes are exactly equal, but with opposite sign because of the r22, −r22 linear EO coefficients. Because of the symmetric nature of the switching characteristics of the balanced bridge interferometer with respect to voltage, both the TE and TM modes would be switched to the upper output port as shown in FIG. 2A when a normalized applied voltage V=Vπ (or V =−Vπ) is applied. Thus, the related art optical switch 100 provides polarization independent switching. However, as shown in FIG. 2B, for effective operation of the related art switch 100, the length of each of the directional coupler sections 142, 144 LB(=½Ldc) must be very precisely manufactured to be equal to ½lc, which is the coupling length of the switch 100. When Ldc(=2 LB) does not equal lc, light entering the upper input channel cannot be switched completely from the output port (=) of the waveguide 120 to the output port (×) of the other waveguide 130. Note that Ldc equals 2×(LB), and LB=length of each 3 dB coupler (input and output). Thus, LB must precisely equal ½lc for a low crosstalk switch. When Ldc does not equal lc, the crosstalk of the switch increases rapidly as Ldc deviates from lc. This is shown in FIG. 2B, for Ldc=0.6lc, 0.8lc, 1.0lc and 1.4lc. Accordingly, when Ldc does not equal 1.0lc precisely, the high crosstalk can make the switch 100 a non-working switch.
As described above, the related art polarization independent optical switches have various disadvantages. Crosstalk of the switch depends on fabricating optimal 3 dB couplers. Ldc precision is limited by fabrication tolerances, and a precise length for the 3 dB couplers is hard to achieve. For example, a 10% variation in coupler length can render an optical switch defective. Thus, a low crosstalk (<−25dB) switch is difficult to achieve. Further, an optimal fabrication parameter to achieve a 3 dB-coupler for the TE mode is often somewhat different than the optimal fabrication parameter required for a 3 dB-coupler for the TM mode. Thus, it is difficult to achieve an exact 3 dB coupling for both TE and TM modes simultaneously. This will result in an undesirable high crosstalk for either TE or TM modes or both.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.